I. Field of the Invention
This invention relates to the formation of a transparent barrier coating on a packaging film, particularly a plastic packaging film, and to the resulting coated film and apparatus for producing the coated film.
II. Description of the Prior Art
Plastic packaging films used in the food industry are normally made moisture and oxygen impermeable by coating the plastic film on one side with a relatively thick layer of aluminum. The resulting film is opaque, so that food contents cannot be seen, and the films cannot be used in microwave ovens because of shorting and reflections caused by the metal layer.
There is a need for transparent, microwavable packaging films having the required barrier properties. While multi-layer plastic film laminates can be used to reduce oxygen and water vapour transmission characteristics of packaging films, satisfactory structures are very expensive and often require as many as six different plastic film layers (see Modern Plastics, August 1986, pp 54-56).
In recent years, a different approach has consisted of vacuum depositing thin films of inorganic coatings onto flexible transparent polymer laminates (see, for example, U.S. Pat. No. 4,702,963 issued on Oct. 27, 1987 to Optical Coating Laboratory Inc. and Japanese Patent Application 60 46,363). A recent article in Paper, Film and Foil Converter, June 1988, pp 102-104, describes the deposition of transparent silica barrier coatings on plastic films via electron beam technology. It is apparent that complex and expensive equipment has to be utilized to deposit such barrier coatings onto plastic substrates and that the resulting coatings may be subject to cracking upon flexing of the film. Furthermore, the silica type films used in the process exhibit a yellowish colouration when laminated with transparent flexible polymer films for use in packaging, and this colouration makes many food contents look unappealing. Finally, materials deposited by electron beam techniques are typically less dense than the bulk form of the coating material and so the barrier properties are not optimal.
Non-porous oxide films produced on certain valve metals by anodization are denser than similar materials deposited by electron beam techniques or other types of deposition. However, while such non-porous, dense anodic oxide films make excellent candidates for use in transparent vapour barrier/polymer composites, such films cannot be easily separated over large areas from the metal on which they are formed. Dissolution of the underlying metal base by chemical means would be a possible approach, but would be highly uneconomical and cumbersome and would be difficult to achieve without the oxides themselves being subject to inadvertent dissolution by such means.